Process for the pickling of metallic materials

ABSTRACT

Method in which the pickling solution is sprayed onto the metal part in a confined atmosphere with an oxygen supply by feeding into the spraying enclosure an oxygen-containing gas, the pickling solution being recycled in a closed pickling solution circuit. The device for pickling metal surfaces comprises a spraying enclosure (1) with a collecting tank (2), means for spraying (6) a pickling solution (S) in said enclosure, means (11) for recirculating said solution between said tank (2) and said spraying means (6) and means (8,9) for passing an oxygen-containing gas through said enclosure, and optionally means (13) for maintaining the temperature of said solution, and control means. The method of the invention is designed to improve the productivity of lines for pickling stainless or alloy steel components while economising on reagents.

The present invention relates to a process for the acid pickling ofmetallic materials, especially ones made of alloy steel, stainless steelor titanium alloys.

According to a process which is known, especially from FR-A-2,587,369,the pickling of steel materials is performed in a bath consisting of anaqueous solution containing ferric ions and hydrofluoric acid. This bathhas the advantage of not containing nitric acid and therefore of notgenerating toxic compounds derived from nitric acid during pickling.

As a substitute for hydrofluoric acid, which may be dangerous to handle,it has been proposed in FR-A-2,657,888 to use organic acids which arenot iron-oxidizing agents, especially formic acid or acetic acid.

Other inorganic acids, such as sulphuric acid or phosphoric acid, mayalso be used in making up such pickling baths.

It is known to carry out the pickling of the surfaces of metalcomponents by immersing the components in baths of the aforementionedtype. During pickling, the baths may be regenerated by introducing anoxidizing agent so as to increase their life. To do this, air or anoxygen-containing gas is bubbled into the bath, or else a more powerfuloxidizing agent than gaseous oxygen, such as ozone, hydrogen peroxide ora compound of the peroxide, peracid or persalt type, may be introduced.

However, especially in the case of the continuous pickling of strip, thepickling of wire or the pickling of components, the pickling kineticsmay be too slow and require the plant to be expensively tied up for anexcessive period of time.

In order to increase the pickling kinetics appreciably, it is known toapply the pickling solution by spraying it onto the surface of the saidmetal component: thus, the solution in contact with the surface to bepickled is continuously being replenished, while at the same time thepickling residues are being removed very rapidly.

According to a conventional process, the pickling solution is sprayedonto the component to be pickled and the solution, after it has comeinto contact with the component, is recovered and recycled in a closedpickling-solution circuit. The solution is usually regenerated bybubbling air or an oxygen-containing gas in at one point in thepickling-solution recirculation circuit.

In industrial pickling processes, these operations are undertaken in aclosed spray chamber in which the components to be pickled lie. Underthese confined-atmosphere pickling conditions, it has been observedthat, although the pickling rate with the said sprayed solution wasinitially much higher than that in immersion pickling, this improvementdisappeared after a few hours and could not be maintained throughout thelife of the said solution, this being so despite the regeneration of thesolution by bubbling air or oxygen in.

The efficacy of the process thus rapidly returns to being comparablewith, or even inferior to, that of an immersion process.

The object of the invention is to provide a process for the-acidpickling of metal components using spraying, in which process thepickling rate is significantly improved throughout the period of use ofthe same solution.

In order to maintain the pickling rate with the said sprayed solution ata high level, it is possible to add a powerful oxidizing agent,especially hydrogen peroxide, to the pickling solution.

However, such oxidizing agents, especially hydrogen peroxide, have thedrawback of being expensive.

The object of the invention is also to improve, in a particularlyeconomical way, the pickling rate in processes of the aforementionedtype.

To this end, the subject of the invention is a process for the picklingof metallic materials, especially ones made of alloy steel, stainlesssteel or titanium alloys, using a pickling solution S containing ferricions in acid medium comprising at least one acid chosen from a haloacid,especially hydrofluoric acid, an inorganic acid, especially sulphuricacid or phosphoric acid, and organic acids which are not iron-oxidizingagents, especially formic acid or acetic acid, in which process the saidsolution is sprayed onto the material, recovered and recycled in aclosed pickling-solution circuit and the said spraying is carried out ina confined atmosphere in a spray chamber with an influx of oxygen byintroducing an oxygen-containing gas G into the said chamber.

The pickling process of the invention may also be applied tozirconium-based, cobalt-based or nickel-based alloys or to carbonsteels.

The materials to be pickled may be in the form of strip, wire, tube,plate or any metal component.

Introducing an oxygen-containing gas into the confined atmosphere makesit possible to maintain, throughout the pickling operation, a highpickling rate. The oxygen contained in the said gas may be introduced,even partially, in the form of ozone.

This influx of oxygen may advantageously be performed continuously, bysustaining the flow of an oxygen-containing gas G in the spray chamber,and very advantageously by uniformly distributing the continuous flow ofthe said gas in the chamber in order to ensure that the composition ofthe atmosphere in the chamber is homogeneous.

Preferably, the oxygen content of the said oxygen-containing gas G isgreater than 10%. The gas may, for example, be air.

Advantageously, the flow rate of the said oxygen-containing gas G is atleast equal to the spray flow rate of the said solution S.

Preferably, the flow rate of the said gas G makes it possible toreplenish the atmosphere in the spray chamber on average at least everyfifteen minutes, and preferably every five minutes.

It is also advantageous for the influx of oxygen to enable an oxygencontent of the atmosphere in the spray chamber at least equal to 10% tobe maintained.

Moreover, the ratio between the spray flow rate of the said solution Sonto the metallic surface, expressed in liters/minute, and the surfacearea to be pickled per unit time, expressed in square meters per minute,is preferably at least equal to five.

In general, it is preferable for the pickling solution to be sprayed inthe form of small droplets which provide a large surface area forexchange between the sprayed liquid and the atmosphere prevailing in thespray chamber. The size of the droplets depends on the flow rates of theliquid and especially of the gas in the said chamber and advantageouslylies within the range corresponding to atomization processes.

Preferably, in the process of the invention, at least part of thepickling solution is introduced into the said chamber by atomization.

The pickling process may be a batch process, the spraying of thecomponents to be pickled taking place in a closed chamber equipped withoxygen introduction means.

However, it is generally preferred to operate continuously in a chamberthrough which the components to be pickled may run, but which creates aconfined atmosphere around the said components. This chamber may, forexample, be a spray tunnel. The chamber is also equipped with oxygenintroduction means.

In an advantageous embodiment of the invention, a pickling solution S isused which contains hydrofluoric acid and Fe³ +ions, present in the formof fluorine complexes, the concentration of ferric ions Fe³⁺ beingbetween 1 and 150 grams per liter, and which has a pH between 0 and 3;it also contains Fe²⁺ ions and its redox potential measured with respectto a saturated calomel reference electrode is between -200 mV and +800mV; such a pickling solution is especially described in European PatentApplication No. 0,188,975 filed by the Applicant.

Surprisingly, without adding a powerful oxidizing agent to the picklingsolution during use, especially hydrogen peroxide, even after many hoursof using the same solution, by recirculating it in a closed circuit, andof spraying the said solution onto components to be pickled in the saidconfined atmosphere, for example in a tunnel, this confined atmospherebeing replenished by virtue of the introduction of an oxygen-containinggas, the pickling rate remains lastingly much higher than the picklingrate which would have been obtained by immersion of the said componentsin the said solution and oxidant regeneration by bubbling oxygen intothe said solution.

During pickling, the consumption and ageing of the pickling solution maybe monitored by measuring the redox potential and the pH of the solutionin the recirculation circuit.

Thus, the pickling process may be operated by monitoring the redoxpotential of the solution. It will be possible to maintain the value ofthe potential at a predetermined value by adjusting the flow rate of gasG introduced, by adding an oxidizing agent to the solution S in therecirculation circuit or by adjusting the spray flow rate of thesolution S.

Monitoring the redox potential of the pickling solution proves to beparticularly advantageous within the context of the present inventionsince it makes it possible to use the recycled pickling solution for avery long period of time with good pickling efficacy, while extendingthe time between interruptions for maintenance of the equipment.

The present inventors have been able to determine that it is possible tocarry out spray pickling using a solution having a redox potential whichis sufficiently high, to ensure good pickling efficacy, but sufficientlylow to limit the precipitation and crystallization of ferric compoundsand thus reduce the risk of clogging in the plant, especially at thespraying elements.

In particular, measuring the redox potential of the pickling solutionmakes it possible, by comparing it with the initial value of thepotential of the solution at the start of pickling, to assess the ageingof the said solution and to detect any anomalies. Thus, although one ofthe aims of the invention is to avoid having to use powerful, butexpensive, oxidizing agents, it may be necessary, depending on the valueof the redox potential, to have a strong oxidation means whichtemporarily and/or locally supplements the action of the oxygen carriergas introduced into the spray chamber in order quickly to return to anadvantageous redox potential allowing good pickling.

Furthermore, the process according to the invention may be used forpickling with solutions containing ferric ions in acid media comprisingacids other than hydrofluoric acid, especially organic acids, used inclosed circuit and therefore requiring regeneration by means of anoxidizing agent, for example by means of an oxygen-containing gas orhydrogen peroxide, this regeneration being controlled by measuring theredox potential of the solution.

It has been possible to show that, in the case in which the redoxpotential is maintained by adding a powerful booster oxidizing agent,especially hydrogen peroxide, to the pickling solution, the process ofthe invention remains economical compared with an immersion process inwhich the same regulation of the redox potential is carried out. This isbecause, for the same efficacy, a smaller quantity of booster oxidizingagent is used in the spraying process of the invention.

Advantageously, the pickling solution is maintained within apredetermined temperature range, preferably at a constant temperature towithin operating variations, in the closed solution-recirculationcircuit.

Advantageously, a heating and/or reheating operation is carried out atsome arbitrary point in the recirculation circuit, using any suitabletechnique. Preferably, a stream of hot oxygen-containing gas,advantageously a stream of hot air, is made to flow into the solution soas simultaneously to reheat and to regenerate the pickling solution. Inthe recirculation circuit, reheating takes place preferably just priorto the spraying of the solution.

The process according to the invention applies to continuous pickling,especially the pickling of metal strip, and to batch pickling,especially pickling of coils of wire or of metal components.

The subject of the invention is also a device for pickling the surfacesof metal components by spraying, which implements a process as describedhereinabove. This device comprises:

a spray chamber provided with a collecting tank;

means for spraying a pickling solution in the said chamber;

means for recirculating the said solution between the collecting tankand the spraying means;

and means for ventilating the said chamber using an oxygen-containinggas.

The device according to the invention may include one or more of thefollowing characteristics:

the said ventilating means deliver the said oxygen-containing gas at aminimum flow rate representing the volume of the said chamberreplenished at least every fifteen minutes, and preferably every fiveminutes;

the said means for spraying the said pickling solution are of thepneumatic atomization type;

the said means for spraying the said pickling solution compriseconventional means of the sprinkling type and means of the pneumaticatomization type.

The device according to the invention may furthermore comprise means formaintaining the temperature of the said solution, possibly providingheating or cooling.

Pickling processes usually employ solutions at a temperature above roomtemperature, these solutions tending to cool down during use. Thetemperature maintenance means are therefore preferably means forreheating the pickling solution.

Reheating means may comprise a conventional vessel reheating byconduction or alternatively means for blowing a hot oxygen-containinggas, especially hot air, into the solution, these two types of meanspossibly being combined. They are preferably arranged just upstream ofthe spraying means.

Advantageously, the device may furthermore comprise liquid/solidseparation means, especially settling means, in order to strip thepickling solution flowing in closed circuit of the solid particlesarising from pickling which are entrained with the solution to thecollecting tank. In this embodiment, the said separation means may beprovided with means for blowing a hot oxygen-containing gas into theregion where the liquid resides.

The device may advantageously comprise monitoring means comprising atleast one measurement probe, especially one for measuring thetemperature, the pH or the redox potential of the solution, these meansbeing installed at any point in the pickling-solution recirculationcircuit.

These monitoring means may comprise, in particular, a probe formeasuring the redox potential of the pickling solution.

In the latter case, the device may furthermore comprise means forinjecting an oxidizing agent, especially hydrogen peroxide, into thecircuit for recirculating the said solution, these means being slaved tothe said probe for measuring the potential.

Moreover, the said ventilating means may be equipped with a valve forregulating the gas flow rate, this valve being slaved to the said probefor measuring the potential.

Likewise, the said spraying means may be equipped with a valve forregulating the flow rate of the solution, this valve being slaved to thesaid probe for measuring the potential.

Illustrative embodiments of the invention will now be described withregard to the appended drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a device for the continuous pickling ofmetal strip using spraying according to the invention;

FIG. 2 is a sectional view of the device in FIG. 1, depicting two spraybars of the pickling device according to the invention;

FIG. 3 is a side view of a device for spraying coils of wire, formingpart of a pickling device according to the invention.

The device depicted in FIG. 1 comprises a spray chamber, means forspraying a pickling solution in the said chamber, means for ventilatingthe said chamber, means for controlling, means for recirculating, meansfor treating and means for maintaining the temperature of the saidsolution.

The said spray chamber comprises a tunnel 1 equipped with a collectingtank 2, a cover 3, an inlet lock 4A and an outlet lock 4B for the stripto be pickled, and means for transferring the said strip, these beingcomposed of rollers 5 which define a strip path or pickling plane P.

As is conventional, the spraying means comprise spray bars 6 which areparallel to the rollers 5, regularly spaced apart and arranged above andbelow the said pickling plane P, as depicted in detail in FIG. 2; thespray bars 6 are rigid tubular pipes which terminate in a multitude ofspray nozzles 7 arranged uniformly along the spray bar 6.

The said spray nozzles 7 are of a type known per se, intended to producea flat jet.

The distance separating the orifices of the nozzles 7 in the picklingplane P in the direction of the jet is between 5 cm and 70 cm, andpreferably equal to approximately 30 cm.

The nozzles 7 of the same stray bar 6 are oriented so as to create flatjets which extend over the width of the strip path and the distancebetween the nozzles 7 is less than the width of the jet in the region ofthe pickling plane P in such a way that the flat jets of each nozzle ofthe same spray bar partially overlap over the entire width of the strippath, as depicted in FIG. 2.

In general, the ventilating means comprise a circuit for supplying gasto the chamber and a circuit for extracting gas from the chamber. Thesecircuits are preferably designed, in a manner known per se, to ensurethat the atmosphere in the chamber has a homogeneous composition.

In the device in FIG. 1, the means for ventilating the tunnel comprise aventilator 8 connected to one end of the tunnel by a gas supply pipe anda gas extraction flue 9 located at the other end of the tunnel.

The inlet of the ventilator 8 is connected to an air intake port, notdepicted, and the ventilator is equipped with a valve, not depicted, forregulating the flow rate; the flue 9 is connected to a gas treatmentdevice, not depicted, which allows recovery of some of the extractedmist or droplets from the gas flux.

The said recirculation means comprise a buffer tank 10 connected to thecollecting tank 2 and a pump 11 for reinjecting, under pressure, thesolution from the buffer tank 10 to each of the spray bars 6.

The pump 11 has characteristics which make it possible to feed the saidspray bars at a pressure of between 0.5×10⁵ and 7×10⁵ Pa, and preferablya pressure equal to 3×10⁵ Pa; the pump 11 is equipped with a valve, notdepicted, for regulating the flow rate.

The buffer tank 10 is equipped with a feed inlet, especially for toppingup with pickling solution S.

The said control means consist of a probe for measuring the redoxpotential, this probe, not depicted, being installed in the region ofthe buffer tank 10.

Without departing from the invention, the said probe for measuring theredox potential may be installed at other points in thepickling-solution recirculation circuit.

The said treatment means comprise a settling tank 12.

The settling tank 12 is installed in a secondary recirculation circuitconnected to the buffer tank 10, the flow rate of which is set by a pump12'; the settling tank 12, known per se and not described in detailhere, is equipped with a means for extracting the solid residues R.

The said means for maintaining the temperature comprise a heater 13which is installed in the pipe connecting the pump 11 to the spray bars6; the heater 13, known per se, is not described in detail here.

A description will now be given of the implementation of the picklingprocess according to the invention using the device describedhereinabove.

The process according to the invention is used here to pickle astainless steel strip B of width Lb.

The buffer tank 10 is filled with a pickling solution S containingferric ions in hydrofluoric acid medium.

The means for spraying, recirculating, treating and heating the picklingsolution are switched on, as are the means for ventilating the spraytunnel 1.

The strip B is made to pass continuously through the spray tunnel 1 onthe rollers 5 at a run speed Vb, in succession through the inlet lock 4Aand then the outlet lock 4B.

Using the valve for setting the pump 11, it is possible to regulate thespray flow rate of the pickling solution in such a way that, expressedin liters per minute, it is at least equal to five times the surfacearea to be pickled, here (Vb×Lb), expressed in m² /minute.

With the total volume of the spray tunnel being V, in m³, the flow rateof- air forced by the said ventilator 8 into the said tunnel is set,using the valve for regulating the flow rate of the ventilator 8, toV/15 m³ /minute, in such a way that the atmosphere in the said tunnel ison average replenished at least every fifteen minutes, and preferablyevery five minutes.

Passing through the spray tunnel 1, and in particular between the spraybars 6, the strip B is sprayed uniformly on both its sides by thepickling solution S sprayed by the nozzles 7 of the spray bars 6.

The spent solution, comprising solid pickling residues in suspension,pours under gravity into the collecting tank 2 then into the buffer tank10.

The probe for measuring the redox potential makes it possible to measurethe redox potential of the solution S in the buffer tank 10.

The solid residues in the solution S are separated by means of thesettling tank 12 when the said solution passes into the secondaryrecirculation circuit connected to the buffer tank 10.

The solution S, stripped of the solid residues in suspension, isreinjected by the pump 11 to the spray bars 6 and the spray nozzles 7,the solution being heated by the heater 13.

The heater 13 allows the temperature of the solution to beadvantageously maintained between 15 and 80° C., and preferably between40 and 70° C.

As the strip B is running progressively through the spray tunnel 1, thesolution S is continuously sprayed onto the said strip in a constantlyoxygen-replenished atmosphere and recirculates in closed circuit in thepickling device.

In order to compensate for consumption of the pickling solution S and tomaintain the pH within a predetermined range of values, it is possibleto add acid or fresh solution directly to the buffer tank via its feedinlet or at other suitable points in the plant.

Without departing from the process according to the invention, it ispossible to maintain the value of the redox potential measured by theprobe at a predetermined constant value by adjusting the air flow rateof the Ventilator 8 or by adding to the buffer tank 10 a boosteroxidizing agent more powerful than air, for example a peroxide, apersalt or ozone, or by adjusting the spray flow rate.

According to one variant of the device of the invention, the spraynozzles 7 are replaced by pneumatic atomization nozzles fed, aspreviously, with liquid by the pump 11 and fed with pressurized gas byan air compressor or an oxygen-containing-gas compressor.

The pneumatic atomization nozzles are known per se and are not describedhere; the characteristics of the pump 11 are tailored to the liquid-feedspecifications of the said nozzles.

Thus, for implementing the process, the solution S is advantageouslysprayed, by atomization, onto both sides of the strip to be pickled andthe atmosphere in the pickling tunnel is advantageously replenished atthe spray means themselves by pneumatic atomization itself.

According to another variant of the device of the invention, the spraytunnel is vertical; this arrangement may be advantageous for thepickling of wire or of metal tube.

According to another variant of the invention, the device is suitablefor pickling coils or reels of wire. It comprises a spray chamberequipped with a support for the coil, means for spraying a picklingsolution onto the coil in the said chamber, means for monitoring, meansfor recirculating, means for treating and means for maintaining thetemperature of the said solution.

FIG. 3 depicts a detail of such a device and illustrates the spraying ofa coil of wire.

The device comprises a support 14 in the form of a hook which supports,on its lower part, a coil of wire C.

The spraying means comprise two spray bars 6 arranged above and belowthe coil of wire C and equipped with spray nozzles 7, similar to thenozzles 7 depicted in FIG. 2 and supplied with pickling solution S via apipe connected to the circuit for recirculating the said solution.

The spray bars 6 are arranged horizontally in the chamber, one above theother, the nozzles 7 of the upper spray bar facing the nozzles 7 of thelower spray bar. They are sufficiently far apart for the coil C carriedby the support 14 to be able to be inserted into the intermediate space.

The power and orientation of the jets produced by the nozzles 7 are suchthat the entire coil is brought into contact with the pickling solution.

This spray device may be integrated in a batch pickling device for coilsof wire, the support 14 being fixed with respect to the closed andventilated pickling chamber. It may also be integrated in a continuouspickling device for coils of wire, which comprises several pairs ofspray bars 6 defining a horizontal space through which moving supports14 carrying coils C pass and in which a pickling solution S is sprayedonto the succession of coils C.

The following examples illustrate the invention.

Comparative Example 1

A steel plate is pickled by immersing it in a pickling bath containingferric ions and hydrofluoric acid.

A 20 mm×50 mm rectangular plate of AISI 430 grade steel is used.

Approximately 1 l of an aqueous pickling solution containing 30 gramsper liter of hydrofluoric acid (expressed as HF), 30 grams per liter oftotal iron in solution and having a redox potential of 250 mV withrespect to a saturated calomel reference electrode, measured at 60° C.,is prepared.

This solution is placed in a pickling tank equipped with a probe formeasuring the redox potential and with means for bubbling air into thebottom of the tank in order to stir the solution in the tank.

The said plate is pickled by immersing it in the tank and air is bubbledthrough it at a flow rate of between 10 and 20 l/minute. Duringpickling, the temperature of the bath is maintained between 50 and 60°C. and the pH is maintained at its initial value by adding hydrofluoricacid.

Measurements are made of the initial weight change of the steel plateevery thirty seconds for two minutes, and then of the average weightchange after four hours, and of the change in redox potential of thesolution as a function of time over four hours, this change beingindicative of the ageing of the solution.

The following results are obtained:

initial weight change: 30 g/m² /min;

average change in 4 h: 3.5 g/m² /min;

change in the redox potential: a reduction of more than 100 mV in 4hours.

Were it to be desirable to increase the life of such a solution further,it would be necessary to supplement the air bubbling with the additionof small amounts of oxidizing agents, for example hydrogen peroxide.

EXAMPLE 1

The same 20 mm×50 mm rectangular plate of AISI 430 grade steel ispickled using a spray device.

Approximately 2 l of the same pickling solution as in ComparativeExample 1 are prepared.

The spraying device takes the form of a spray tunnel and comprises achamber provided with a cover and a tank filled with the picklingsolution S and equipped with a probe for measuring the redox potential.

Installed in this chamber is a sprinkling nozzle fed with the spraysolution by a pump whose inlet pipe dips into the said tank.

The plate to be pickled is placed horizontally above the tank,approximately 30 cm below the sprinkling nozzle.

A ventilator continuously aerates the spray chamber.

The pickling solution S is sprayed uniformly over the entire surface ofthe plate; the spent solution is collected by the tank under the plate;the pump extracts the collected solution and sends it back to thesprinkling nozzle. The temperature of the solution is maintained between50 and 60° C. and the pH is maintained at its initial value by addinghydrofluoric acid.

As in the case of Comparative Example 1, the initial weight change ofthe steel plate is measured every thirty seconds for two minutes, andthen the average weight change at the end of four hours and the changein the redox potential of the solution as a function of time over fourhours are measured.

The following results are obtained:

initial weight change: 50 g/m² /min;

average change in 4 h: 8.7 g/m² /min;

change in the redox potential: reduction of less than 80 mV in 4 hours.

Compared to Comparative Example 1, the solution S has a much longer lifewithout adding an oxidizing agent more oxidizing than air, such ashydrogen peroxide.

Furthermore, by virtue of this spraying process, on the one hand animprovement in the pickling rate is obtained during the initial periodof use of the pickling solution (in this case: 2 minutes) and, on theother hand, this improvement in the pickling rate remains significanteven after 4 hours.

Compared to the immersion pickling in Comparative Example 1, the overallpickling kinetics are multiplied by approximately a factor of 2.5.

Comparative Example 2

The conditions for this test are identical to those in ComparativeExample 1, except that the plate is pickled for 3 hours, keeping theredox potential of the pickling solution constant and at its initialvalue; to do this, a hydrogen peroxide solution containing 30% by weightof H₂ O₂ is added regularly to the pickling solution.

At the end of 3 hours, the total volume V₀ of hydrogen peroxide added tothe pickling solution, in order to keep its redox potential constant, ismeasured; the weight change of the steel plate is also measured.

The average weight change in 3 hours is 10 g/m² /min.

EXAMPLE 2

The conditions for this test are identical to those in ComparativeExample 2, except that the plate is pickled for 3 hours, keeping theredox potential of the pickling solution constant and at its initialvalue; to do this, a hydrogen peroxide solution containing 30% by weightof H₂ O₂ is added regularly to the pickling solution.

At the end of 3 hours, the total volume V of hydrogen peroxide added tothe pickling solution, in order to keep its redox potential constant, ismeasured; the weight change of the steel plate is also measured.

The average weight change in 3 hours is 14 g/m² /min.

In order to assess the difference in efficacy of the processes inComparative Example 2 and Example 2, the volumes V₀, V of hydrogenperoxide added are expressed relative to the amounts of steel pickled,in order to calculate the saving in hydrogen peroxide, which in thiscase amounts to 54%.

Thus, in the case in which an oxidizing agent such as hydrogen peroxideis used, the oxygen-jet process of the invention remains economicalsince it makes it possible to decrease by half the amount of thisexpensive reagent for achieving the same efficacy.

Comparative Example 3

In this example, a 20 mm×50 mm rectangular plate of AISI 304 grade steelis pickled by immersion under identical conditions to those inComparative Example 1. The following results are obtained:

initial weight change: 27 g/m² /min;

average change in 4 h: 0.5 g/m² /min.

EXAMPLE 3

In this example, the same plate is pickled by spraying, under identicalconditions to those in Example 1. The following results are obtained:

initial weight change: 33 g/m² /min;

average change in 4 h: 1 g/m² /min.

Compared to the immersion pickling in Comparative Example 3, the averagepickling kinetics over several hours are multiplied by 2 in the processof Example 3.

EXAMPLE 4

The pickling solution used in the above examples may be used for thecontinuous pickling, in a spray tunnel, of a strip of AISI 304 gradesteel.

The spray tunnel has a total volume of 50 m³. It is equipped at one ofits ends with a ventilator, connected to an air intake, blowing air intothe tunnel with a flow rate of 3.5 m³ /minute.

The steel strip, having a width of 1.3 m, runs through the tunnel at aspeed of 25 m/min.

The spray bars, of a standard type, are equipped with spray nozzlesspaced 30 cm apart.

The pickling solution pumped into the collecting tank of the tunnelfeeds the spray bars at a pressure of 3×10⁵ Pa for a flow rate of 2 m³/min.

A heater keeps the temperature of the pickling solution between 50 and60° C.

This pickling device was able to be operated for periods of severalweeks, maintaining a satisfactory redox potential with good picklingquality.

EXAMPLE 5

An aqueous pickling solution containing 60 g/l of hydrofluoric acid(expressed as HF), 70 g/l of total iron in solution and having a redoxpotential of 220 mV with respect to a saturated calomel referenceelectrode, measured at 75° C., is prepared.

This solution is used for the continuous pickling of cold-rolled AISI304 grade stainless steel strip in a spray tunnel having a volume ofapproximately 31 m³.

This tunnel is equipped with three ports connected to an air intakedevice and the flow rate of air drawn in is 160 m³ /min; the three portsare arranged at regular intervals along one of the side walls of thechamber, parallel to the path along which the strip runs.

The tunnel, is equipped with 32 spray bars arranged approximately 25 cmabove and below the running strip and distributed at regular intervalsalong the tunnel; each spray bar is equipped with spray nozzlesuniformly spaced apart by 35 cm.

The spray bars are fed with pickling solution at a pressure of 3×10⁵ Pa(3 bar) and deliver in total 2 m³ /min of solution.

A heater makes it possible to maintain the pickling solution atapproximately 75° C. The redox potential of the solution is maintainedat 220 mV by regularly adding hydrogen peroxide. The steel strip, havinga width of 1.24 m runs through the tunnel at a speed of 48 m/min.

The results obtained with this plant were compared with those of animmersion pickling plant having the same length with the same solutionmaintained at the same temperature and at the same redox potential,using hydrogen peroxide, and applied to the same steel strip running ata slower speed of 42 m/min.

It is observed that, by virtue of the invention, 15% less hydrogenperoxide is consumed for a 15% greater run speed.

There is therefore a 15% gain in pickling efficacy and a 30% saving inthe amount of hydrogen peroxide used for pickling a given length ofsheet.

We claim:
 1. A process for pickling a metallic material,comprising:spraying onto said metallic material a pickling solution; andrecovering and recycling said pickling solution; wherein said spraying,recovering and recycling are carried out in a closed pickling-solutioncircuit, said spraying is carried out in a confined atmosphere in aspray chamber with an influx of oxygen by introducing anoxygen-containing gas into the chamber, and said pickling solutioncomprises ferric ions and at least one acid selected from the groupconsisting of a haloacid, an inorganic acid and, an organic acid whichis not an iron-oxidizing agent, wherein said metallic material is analloy steel, stainless steel or titanium alloy.
 2. Process according toclaim 1, in which the flow of the oxygen-containing gas in the spraychamber is sustained.
 3. Process according to claim 1, in which theoxygen content of the oxygen-containing gas is greater than 10%. 4.Process according to claim 1, in which the flow rate of theoxygen-containing gas is at least equal to the spray flow rate of thesolution.
 5. Process according to claim 1, in which the flow of theoxygen-containing gas makes it possible to replenish the atmosphere inthe spray chamber on average at least every 15 minutes.
 6. Processaccording to claim 1, in which the oxygen content of the atmosphere ismaintained above 10%.
 7. Process according to claim 1, in which theratio between the spray flow rate of the solution, expressed in litersper minute, and the surface area to be pickled per unit time, expressedin square meters per minute, is at least equal to five.
 8. Processaccording to claim 1, in which at least part of the pickling solution isintroduced into the chamber by atomization.
 9. Process according toclaim 1, in which the redox potential of the pickling solution ismaintained at a predetermined value.
 10. Process according to claim 9,in which the redox potential is set to the predetermined value byadjusting the flow rate of the introduction of the oxygen-containinggas.
 11. Process according to claim 9, in which the redox potential isset to the predetermined value by adding an oxidizing agent to thepickling solution.
 12. Process according to claim 9, in which the redoxpotential is set to the predetermined value by adjusting the spray flowrate of the solution.
 13. Process according to claim 1, in which astream of hot oxygen-containing air is blown into the pickling solution.